Despite the importance of the effects of aging on functional capacity and quality of life, little is known about the causes of individual differences in aging. It is generally agreed that the most powerful strategy to untangle genetic and environmental influences in the development of human characters is the adoption design, and that the most powerful adoption design is the study of twins reared apart. An unprecedented opportunity for the study of an elderly population has arisen in Sweden where a large sample of elderly twins who were reared apart has been identified in the Swedish Twin Registries. The proposed project will consist of two phases. In Phase I, we shall obtain data from approximately 2800 twin pairs (reared apart and matched sample reared together by mail-out questionnaires) on personality, health, attitudes and specific environmental circumstances. Phase II will involve individual testing of those twins of 50 - 80 years of age whose age at separation and completeness of separation make them most appropriate for more intensive study. The testing of Phase II will include measures of cognitive ability, with systematic sampling of the domain of fluid and crystallized intelligence, current health status, specific biomarkers including blood pressure and forced vital capacity, family and social environments and significant life events. This proposed program should provide valuable information on individual differences in these variables in an adult population, the interrelationships among the variables and the relationship (by cross-sectional assessment) of age differences in the means, variances and covariances of the variables and in the relative contributions of genetic and environmental sources of variance and covariance.